Temperature controlled soldering iron employing a variable resistance heating element for temperature sensing

Abstract

A temperature controlled soldering iron with a heating element having a positive temperature coefficient arranged as one leg of a Wheatstone resistance bridge such that the voltage across the heating element signals the temperature of the soldering iron. A comparator is connected across opposite nodes of the bridge and turns off a switching element to cut current flow through the bridge when the heating element is above the desired temperature. A pulse generator continuously supplies pulses to briefly restart current flow through the bridge and allow the temperature to be sensed. The pulses are short duration and occur regardless of whether the comparator has previously turned off current flow, and independent of the temperature of the heating element. After each pulse the comparator controls current flow by leaving the flow on or turning it off according to the temperature of the heating element as measured by comparison of the voltages on the opposite nodes of the bridge.

We claim: 
     
       1. A temperature controlled soldering iron comprising: a heating element having a resistance which varies as a function of temperature, mounted in close thermal contact with a soldering iron tip;   a Wheatstone resistance bridge including the resistive heating element as one leg thereof;   a DC power supply for supplying power to the bridge and to the heating element therein;   a switch means connected to the bridge and having a control lead for switching on and off current flow through the bridge;   a comparator having two inputs connected to opposite nodes of the bridge and an output connected to the control lead of the switch means, the comparator acting to turn off the switch means when the heating element is above the desired temperature as indicated by the relative voltage of the input nodes or to leave on the switch means when the heating element is below the desired temperature; and   a pulse generator continuously generating brief pulses at prefixed time intervals and connected to the control lead of the switch means to turn on the switch means at the prefixed time intervals regardless of whether the comparator has previously turned off the switch means and then leave control of the switch means to the comparator until the next time interval.   
     
     
       2. A temperature controlled soldering iron according to claim 1 wherein one of the resistive legs of the bridge is a variable resistance such that varying the variable resistance adjusts the temperature of the soldering iron. 
     
     
       3. A temperature controlled soldering iron according to claim 1 wherein the switch means is a transistor. 
     
     
       4. A temperature controlled soldering iron according to claim 1 further including a warning lamp connected to the bridge so that the lamp is on when the switch means is on and power is being supplied to the heating element. 
     
     
       5. A temperature controlled soldering iron according to claim 1 wherein the heating element has a positive temperature coefficient.